Is $\mathrm{RuO_2}$ an altermagnet?
In plain words
$\mathrm{RuO_2}$ was the first proposed metallic altermagnet, but muon and neutron measurements find no magnetic order in bulk crystals. Signals reported in thin films might come from surfaces or defects.
Precise statement
Determine whether stoichiometric rutile $\mathrm{RuO_2}$ has collinear antiferromagnetic order with Ru moment above $10^{-3}\,\mu_B$, in bulk and in epitaxial films; if not, identify the origin of the anomalous Hall, spin-splitting torque and spin-resolved ARPES signals reported in films. An answer is a moment value or upper bound per sample type and a mechanism for the film signals.
What would settle it
Muon, neutron and resonant x-ray measurements on the same films used for transport, combined with depth-resolved magnetic probes.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Muon spin rotation and neutron diffraction (2024, arXiv:2405.10820) bound the bulk moment near $10^{-4}\,\mu_{\mathrm{B}}$ per Ru; a 2026 preprint (arXiv:2604.10659) attributes film signals to surface ferrimagnetism.
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